The present invention relates to an optical pickup for optically writing and reading information by projecting light onto an optical recording medium and detecting the light reflected therefrom, and particularly, to an optical pickup which can be subminiaturized.
Along with the practical use of information processing apparatuses for optically processing information, demand has recently increased for a lightweight and miniaturized optical pickup assembly which is the heart of such an apparatus. A lightweight and miniaturized optical pickup is preferable because it enables high-speed processing of information as well as the miniaturization of the information processing apparatus as a whole.
In the optical pickup, its weight and volume can be contracted by reducing the number of optical elements, which are disposed on an incident light traveling path from the light source of an optical system to an optical recording medium and on a reflecting light traveling path From the optical recording medium to a photodetector, so as to have as few as possible, and by optimizing their arrangement.
In general, such an optical pickup, as shown in FIG. 1, is constructed to have a light emitting portion A made up of elements located along a light path from a light source 1 for generating light to an optical disk 5 via an objective lens 4, and a light receiving portion B having a plurality of elements placed along a light path from optical disk 5 to a plurality of photodetectors 7, 8 and 9. Here, a light splining prism 2 of light emitting portion A is to split the light of light source 1 so as to converge part of the light onto a photodiode 3 to be used as a monitor. A light splitting prism 6 of light receiving portion B is to divide and transmit the reflected light of prism 6 to photodetectors 7, 8 and 9. Since light source 1 and the photodetectors 7, 8 and 9 are disposed far apart from each other, such an optical pickup using the light splitting prism 2 and 6 needs ample space and is therefore hard to miniaturize.
Japanese Laid-open Patent Publication hei 5-144031 discloses an optical pickup in which a plate having two parallel half mirrors is used to dispose one of the photodetectors closer to the light source, thereby miniaturizing the optical pickup. Since, however, another photodetector is still far from the light source this technology has a limit in its miniaturizing of the optical pickup.
In addition, for the miniaturization of an optical pickup, a technology using a hologram device has been known (though specific references are not cited herein). A hologram device performs a wave-plane conversion function in which one wave inducts the other between two waves used during the recording of a hologram pattern. As an alternative to the light splitting prism, the hologram device is particularly advantageous in that it can form the path of the reflected light at a slant with respect to the path of the incident light and therefore can reduce the occupied space and number of elements of the optical pickup. In the hologram lens, however, since the change of position of light passing the hologram is great according to the wavelength change of the used light, errors take place frequently. This impedes the practical use of holograms.